It is well established in animals and humans that the maintenance of normal spermatogenesis is critically dependent upon the presence of androgenic steroids within the testis. Studies in the rodent have shown that intratesticular testosterone concentrations are normally thirty-fold higher than serum concentrations, and that levels of intratesticular testosterone ten-fold greater than serum are required to maintain spermatogenesis. It is incredible that the concentration of testosterone in the normal human testis is not known. The long-term objectives of this project are to study the relationship between intratesticular testosterone concentration and spermatogenesis in humans, and to understand the role that inadequate intratesticular testosterone concentration might play in male infertility. The intratesticular concentration of testosterone, its precursors and metabolites, in normal fertile men will be determined by examining testicular fluid obtained by direct percutaneous aspiration from fertile men undergoing vasectomy. Biologically available intratesticular testosterone will be estimated by simultaneously measuring the intratesticular androgen binding proteins, steroid hormone binding globulin (SHBG) and androgen binding protein (ABP). The range of the minimally necessary intratesticular concentration of testosterone required for the maintenance of quantitatively normal spermatogenesis will be estimated by measuring the intratesticular testosterone concentration of men receiving contraceptive doses of testosterone and men with partial hypogonadism prior to their receiving testosterone replacement therapy. The range of intratesticular testosterone concentration in infertile men diagnosed with idiopathic infertility and with varicoceles will be determined. Additionally, the pathophysiological mechanism for abnormally low intratesticular testosterone concentration will be studied in subfertile men. We will directly test the hypothesis that reduced intratesticular testosterone concentration is responsible for infertility by examining the correlation between intratesticular testosterone concentration and improvement in spermatogenesis in infertile men receiving therapies designed to increase intratesticular testosterone concentration.